Summary
Amino acids are deaminated and their carbon skeletons enter respiration, with nitrogen excreted as urea.
In depth
Transamination moves amino groups to alpha-ketoglutarate to form glutamate; oxidative deamination releases ammonia, converted to urea. Carbon skeletons become glucose (glucogenic) or ketone bodies (ketogenic).
Proteins are used for fuel in starvation. Ammonia is toxic and converted to urea. Branched-chain amino acids are used by muscle.
Key idea: protein is a last-resort fuel.
Proteins are normally used for building and repair, but in starvation, high intake, or intense exercise amino acids are used for energy. Proteins are digested to amino acids, which lose their amino group by transamination and oxidative deamination in the liver.
The carbon skeletons enter metabolism as pyruvate, acetyl-CoA or Krebs cycle intermediates, and glucogenic amino acids can form glucose. The released ammonia is toxic and is converted to urea by the urea cycle, then excreted by the kidney (urea is mostly made by mammals; fish excrete ammonia, birds uric acid). Protein gives about 17 kJ per gram.
Example: in prolonged fasting, muscle breaks down, giving amino acids for gluconeogenesis.
Frequently asked questions
What is Protein Catabolism?
Amino acids are deaminated and their carbon skeletons enter respiration, with nitrogen excreted as urea.
Which area of biology covers Protein Catabolism, and how advanced is it?
Protein Catabolism is one of the 53 topics in Metabolism & Bioenergetics. It is rated Intermediate, so it suits learners who already know the core vocabulary. Related topics in this area include Gluconeogenesis, Ketone Body Metabolism, Anabolism and Catabolism.
Why does Protein Catabolism matter?
Kidney or liver disease affects nitrogen handling.
Can you give an example of Protein Catabolism?
During prolonged starvation, muscle protein is broken down.
What should I learn before Protein Catabolism?
Work through the Beginner topics in Metabolism & Bioenergetics first, such as Metabolism, Anabolism and Catabolism, ATP, then return to Protein Catabolism. Intermediate topics assume the Beginner vocabulary.
What topics are related to Protein Catabolism?
The closest topics to Protein Catabolism are Gluconeogenesis, Ketone Body Metabolism, Anabolism and Catabolism, Beta-Oxidation. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Protein Catabolism?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Protein Catabolism. To practise the whole of Metabolism & Bioenergetics, use the Metabolism & Bioenergetics quiz.
How is Protein Catabolism different from Gluconeogenesis?
Protein Catabolism: Amino acids are deaminated and their carbon skeletons enter respiration, with nitrogen excreted as urea. Gluconeogenesis: Synthesis of glucose from non-carbohydrate precursors such as lactate and amino acids, mainly in the liver. Read both side by side and note what each one explains.
How should I study Protein Catabolism step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with Gluconeogenesis and explain the difference aloud. Finally, take the Metabolism & Bioenergetics quiz and come back to this page for any question you miss.